I was completely shocked today to see the discount for APD for kids on flights departing the UK. In a couple of cases you ended up paying 1p for a seat for anyone under 16. This works for any seat under 12,00 GBP.
An example
FR3252 London Stansted to Dusseldorf Weeze. 19 Jan 2017
Adult fare 9,99
Child fare(age 15) 9,99-9,98
Total fare 10.00
Although you still must pay for the one seat,even in this day and age, Ryanair do 1p flights